Aqueous solutions, plastics or glass can be foamed. Recent decades have seen repeated efforts aimed at foaming metals as well and at producing novel foams that have a novel property spectrum due to the combination of the typical foam morphology with the known advantages of metallic materials. Metal provides elasticity, strength and temperature resistance while foam provides low weight, damping, high porosity and a large specific surface area.
Metal foam is a novel material with a systematically created pore structure, it is non-combustible and exhibits great strength. Foams made of metal are airy materials that are lightweight, stiff and yet flexible and that absorb a great deal of energy in case of a crash. Metal foam can also fulfill a wide array of other technical tasks and is particularly suitable for applications such as thermal insulation, noise and vibration attenuation or as a compression element.
Metal foams can consist of up to 85 percent air and a mere 15 percent metal, which makes them very lightweight. They look like conventional synthetic foams but are much stronger. Up until a few years ago, the production methods were too laborious, too costly and too difficult to control, and consequently the results were rarely reproducible. In the meantime, however, melt and powder-metallurgical methods exist that promise to deliver a high quality of the foamed metal. Several methods are known and commonly used for the production of metal foams. For example, a slip is prepared at room temperature in order to make steel foam out of steel powder, water and a stabilizer. Phosphoric acid is added as a binder and foaming agent to this mixture. Two reactions then take place in the slip, leading to the formation of a stable foam structure. On the one hand, the reaction between the steel powder and the acid generates hydrogen gas bubbles that bring about the foaming. On the other hand, a metal phosphate is formed whose adhesive effect solidifies the pore structure. The foam thus created is dried and subsequently sintered without generating any pollutants to form a metallic composite.
A melt-metallurgical method is described, for example, in European patent application EP 1 288 320 A2, in which gas bubbles are introduced into a melt. In order to do so, at least one gas feed pipe with a defined gas outlet cross section protrudes into the melt and individual bubbles are blown into the melt through this pipe. The size of the bubbles is controlled by the setting of the inflow parameters of the gas.
European patent application EP 1 419 835 A1 describes a method and a device for the production of flowable metal foam with a monomodal distribution of the dimensions of the void spaces, likewise based on a melt-metallurgical method. In this context, at least two adjacent feed pipes that are similarly dimensioned and positioned at a defined distance from each other protrude into a metallurgical vessel containing a foamable metal melt. Bubbles are formed in the areas of the protruding pipe ends, whereby a contiguous foam formation is created when areas of the bubble surfaces come to lie against each other and partition walls containing particles are formed.
A drawback of these melt-metallurgical methods is that a metal melt cannot be foamed in its pure state. In order to make the metal melt foamable, it has to be mixed with an agent that increases the viscosity, for example, an inert gas (GB 1,287,994) or with ceramic particles (EP 0 666 784 B) before the foaming is carried out. Only the metal foam that accumulates on the melt surface can flow. Even though this is favorable when it comes to shaping the metal foam, the insufficient stabilization of the metallic walls can lead to a partial collapse of the formed metal foam and thus to the uncontrollable formation of dense zones inside an object produced in this way. Moreover, some of the formed bubbles or the dissolved gas can escape from the melt while the latter is solidifying, so that the released gas is no longer trapped in the melt, resulting in a low porosity of the objects made by means of this method. Moreover, the incorporation of the gas bubbles into the melt requires complex equipment.
A powder-metallurgical method for the production of porous metal objects is described in German patent DE 101 15 230 C2, in which a mixture of a gas-cleaving powder containing a foaming agent and a pulverulent metallic material containing at least one metal and/or a metal alloy is compacted to form a semi-finished product. This semi-finished product is foamed under the effect of heat, a process in which a powder containing a foaming agent is used whose temperature of maximum decomposition is less than 120 K below the melting temperature of the metal or the solidus temperature of the metal alloy. For purposes of producing metal parts having an internal porosity, international patent application WO 2005/011901 A1 describes to first create a foamable semi-finished product consisting of metal and at least one foaming agent that releases gas at an elevated temperature, whereby the metal forms an essentially closed matrix into which foaming agent particles are embedded. The quality of the metal object produced is supposed to be enhanced with a semi-finished product in which the metal matrix that traps the foaming agent particles is formed by the diffusion-welding and/or pressure-welding of metal particles. Towards this end, in a first step, metal particles and at least one agent that releases gas(es) at an elevated temperature, so-called foaming agents, are mixed together, after which, in a second step, the mixture is shaped under elevated pressure and elevated temperature to form a semi-finished part that is allowed to cool off or is cooled down to a temperature below the decomposition or outgassing temperature of the foaming agent while the application of pressure is maintained. In a third step, the semi-finished product is heated to above the decomposition temperature of the foaming agent and, with the creation of internal porosity, the semi-finished product is shaped into a metal foam part.
Another method for the production of metal foam objects is described in international patent application WO 2004/063406 A2. This method can be employed as a powder-metallurgical method or as a melt-metallurgical method. With this solution, a feed material is melted under atmospheric pressure in an open melting vessel without excess-pressure devices and gas is introduced into the liquid phase of the feed material at the same time and/or subsequently, so that the introduction of foaming agent or gas sufficiently provides the melt with gas in order to form a metal foam object having a low density when the melt solidifies. According to the described solution, this effect can be beneficially utilized to produce a metal foam object that has the desired shape if the liquid metal is first placed into a mold and then allowed to solidify in it under ambient pressure that is reduced, at least at times. Due to the solidification of the melt at a reduced ambient pressure, preferably 0.03 bar to 0.2 bar, numerous gas bubbles are formed in the melt but these become trapped in it due to the onset or continuation of the solidification of the melt so that metal foam objects produced in this manner have a low density.
Japanese publication JP 01-127631 (Abstract) likewise describes a method in which, analogously to the above-mentioned solution, hydrogen, nitrogen and oxygen are introduced under atmospheric pressure into the liquid metal or else foaming agent particles such as nitride, hydride or oxide release gas into the melt by means of thermal cracking. The liquid metal mixed with gas is placed into a shaping mold and kept for a certain period of time at a reduced pressure of 400 to 760 mmHg.
High-quality metal foam objects can be created by such powder-metallurgical methods. However, these methods are extremely complex in terms of the material employed and the equipment needed since they call for at least two powder components, namely, metal particles and foaming agent particles. Also, the individual powder components have to be thoroughly mixed prior to any heating and the powder grains have to be sintered together, for instance, by hot isostatic pressing, in order to obtain pores with the best possible homogeneous distribution in the finished metal foam objects. Another drawback lies in the fact that gas already escapes from the foaming agent particles prior to the melting of the metal and then it accumulates in cracks, flaws, etc. This gives rise to pores that are of different sizes and irregularly distributed in the metal foam. The pore size and the volume expansion are difficult to control during the process.